Many researches are under way for commercialization of a dye-sensitized solar cell. Examples include technical development for synthesis of TiO2 nanoparticles of various structures and for increasing the surface area thereof, development of new metal oxide electrodes, development of dyes and electrolytes having high efficiency and stability, and improvement of preparation processes. In particular, development of new photosensitizing dye materials that directly affect the efficiency of dye-sensitized solar cells is actively ongoing. Especially, in order to improve the overall efficiency of a dye-sensitized solar cell, improvement of the light harvesting efficiency of the dyes is important. As examples of such dyes, there are ruthenium (Ru) complexes, porphyrins, phthalocyanines and other various organic dyes.
Among them, porphyrins are actively studied because of their interesting photophysical and chemical properties. Since adequate arrangement of porphyrins leads to many interesting molecules useful as from sensors to new photoelectric materials, there are many efforts to establish defined oligomeric arrangement of porphyrins, especially, that mimics the naturally occurring photosynthetic antenna system. To provide desirable photoelectric properties in an artificial light-harvesting complex as the nature fully utilizes organized pigments in the light-harvesting complex, a higher-level arrangement of porphyrin dyes is necessary.
Up to now, the researches on the interaction between porphyrins have mostly focused on two π-planes arranged side-by-side. In contrast, there have been few researches on the face-to-face arrangement of porphyrins, probably because of the inability to synthesize an effective scaffold.
The development of artificial light-harvesting systems thus far has mainly been accomplished by binding a dye to a biopolymer or synthetic polymer material as a scaffold and forming a complex through self-assembly or binding on the surface of nanoparticles. However, the systems synthesized in this manner are mostly polydisperse complexes and it is not easy to precisely control the position where the dye is attached.
To solve this problem, an artificial light-harvesting system using a dendrimer as a scaffold was proposed. The light-harvesting system using the dendrimer has been used in many multi-pigment array light-harvesting systems because it is monodisperse and has a precisely-defined structure.
However, there are problems that the synthetic yield of the dendrimer is low and it is difficult to arrange two different dyes at desired positions of one dendrimer molecule (non-patent document 1).
Use of the face-to-face arrangement of porphyrins as a scaffold has never been reported.